An internal combustion engine operating at a lean air-fuel ratio and having a NOx catalyst device for purifying NOx arranged in the exhaust passage thereof, has been known. The NOx catalyst device favorably absorbs NOx from the exhaust gas of an engine running under a lean air-fuel ratio of a high oxygen concentration but is not capable of absorbing NOx limitlessly. Therefore, it is necessary to regenerate the NOx catalyst device before the NOx catalyst device is saturated with NOx which has been absorbed by lowering the oxygen concentration in the combusted gas flowing into the NOx catalyst device in order to release absorbed NOx and purify the released NOx by reducing substances such as uncombusted HC and CO contained in the combusted gas.
The regeneration treatment comprises mixing additional fuel (HC) into the combusted gas of a lean air-fuel ratio in the cylinder or in the exhaust passage, and feeding them to the NOx catalyst device. Part of the additional fuel fed to the NOx catalyst device is combusted (oxidized) consuming oxygen in the combusted gas due to the oxidizing catalyst carried by the NOx catalyst device, and lowers the oxygen concentration in the combusted gas in the NOx catalyst device. The remainder of the additional fuel is used for reducing and purifying NOx released from the NOx catalyst device due to a decrease in the oxygen concentration.
Upon feeding the additional fuel in a suitable amount to the NOx catalyst device, it is possible to lower the oxygen concentration to a sufficient degree by consuming much of the oxygen in the combusted gas in the NOx catalyst device, to favorably reduce and purify NOx released from the NOx catalyst device, and to decrease the amount of fuel which passes through the NOx catalyst device to a sufficiently low level.
When additional fuel is fed in an amount larger than a suitable amount to the NOx catalyst device, the result is an increase in the amount of fuel that is neither combusted in the NOx catalyst device nor used for reducing and purifying the released NOx, i.e., an increase in the amount of fuel which simply passes through the NOx catalyst device. Thus, an increase in the amount of fuel which simply passes through the NOx catalyst device is detrimental in regard to not only fuel efficiency, but also the emission of exhaust gas.
There have been proposed devices for purifying exhaust gas of internal combustion engines by arranging a NOx concentration sensor on the downstream of the NOx catalyst device to detect the concentration of NOx flowing out from the NOx catalyst device at the time of regeneration treatment (e.g., see JP-A-5-113116 and JP-A-2004-251134). In the above devices for purifying exhaust gases, if the detected NOx concentration is lower than a preset range, then oxygen concentration has been sufficiently lowered in the combusted gas in the NOx catalyst device and NOx released in sufficient amounts can be reduced and purified to a sufficient degree. At this moment, additional fuel has been fed in an amount larger than a suitable amount; i.e., the fuel simply passes through the NOx catalyst device in a relatively large amount, and therefore, additional fuel is fed in a decreased amount in the next regeneration treatment. Further, if the NOx concentration is higher than the preset range, then the oxygen concentration has been sufficiently lowered in the combusted gas in the NOx catalyst device and NOx has been released to a sufficient degree, but could not be reduced and purified to a sufficient degree. Namely, additional fuel has been fed in an amount smaller than a suitable amount, and the amount of additional fuel is increased in the next regeneration treatment.
In the above device for purifying exhaust gas, when the concentration of NOx flowing out from the NOx catalyst device is lower than a preset range during the regeneration treatment, it is probable that the amount of additional fuel has abnormally decreased due to a defective fuel feeding device, and therefore, the oxygen concentration in the combusted gas is not lowered to a sufficient degree and NOx is not fully released. In the case of such an abnormal decrease, it is no meanless to try to decrease the amount of the additional fuel in the next regeneration treatment.
It is, therefore, an object of the present invention to provide a device for purifying exhaust gas of an internal combustion engine which is capable of deciding an abnormal decrease in the amount of the additional fuel fed during the regeneration treatment by using a NOx concentration sensor arranged on the downstream of the NOx catalyst device.